Aluminum nitride bulk crystals having high transparency to ultraviolet light and methods of forming them

ABSTRACT

In various embodiments, methods of forming single-crystal AlN include providing a substantially undoped polycrystalline AlN ceramic having an oxygen concentration less than approximately 100 ppm, forming a single-crystal bulk AlN crystal by a sublimation-recondensation process at a temperature greater than approximately 2000° C., and cooling the bulk AlN crystal to a first temperature between approximately 1500° C. and approximately 1800° C. at a first rate less than approximately 250° C./hour.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/731,790, filed Mar. 30, 2007, now U.S. Pat. No. 8,012,257,which claims the benefit of and priority to U.S. Provisional ApplicationSer. No. 60/787,399, filed Mar. 30, 2006. The entire disclosure of eachof these applications is hereby incorporated by reference.

GOVERNMENT SUPPORT

This invention was made with United States Government support under70NANB4H3051 awarded by the National Institute of Standards andTechnology (NIST). The United States Government has certain rights inthe invention.

FIELD OF THE INVENTION

The technology disclosed herein relates generally to semiconductorcrystals, in particular semiconductor crystals having high transparencyto ultraviolet light.

BACKGROUND

Semiconductor materials exhibit controllable optical and electricalproperties, such as conductivity, over a wide range. Such control isenabled by use of dopants, which are impurities intentionally introducedinto the crystalline lattice of the semiconductor material to serve assources of electrons (negative charges) or holes (positive charges).Controllable doping enables the fabrication of a wide range ofsemiconductor devices, e.g., light-emitting diodes (LEDs), lasers, andtransistors.

Nitride-based semiconductors such as gallium nitride (GaN) and aluminumnitride (AlN) are of great interest technologically, in part because oftheir wide bandgaps. Controllable and repeatable doping of thesematerials enables the fabrication of light-emitting devices, such asLEDs and lasers, that emit light at short wavelengths, i.e., at blue,violet, and even ultraviolet (UV) wavelengths. Moreover, n- and p-typenitrides can be utilized in the fabrication of transistors suited forhigh power and/or high temperature applications. In an n-typesemiconductor, the concentration of electrons is much higher than theconcentration of holes; accordingly, electrons are majority carriers anddominate conductivity. In a p-type semiconductor, by contrast, holesdominate conductivity.

AlN has a relatively large bandgap of 6.1 electron volts (eV) at roomtemperature, and few dopants for AlN have shallow enough energy levelsin the bandgap to facilitate high electrical conductivity with onlymoderate dopant concentrations. Thus, dopant concentrations often needto be relatively high in order to achieve technologically usefulconductivity levels. Unfortunately, achieving high dopant concentrationlevels in AlN can be difficult. AlN is typically grown at very hightemperatures, making it difficult to incorporate high levels of desireddopants in a controlled way while avoiding the introduction of unwantedimpurities and other point defects. These will introduce deep levels inthe bandgap that counteract the desired effect of the dopant. (That is,the undesired defects will introduce deep levels that will absorb theelectrons or holes introduced by the dopants.) In particular, undertypical growth conditions, oxygen appears to introduce a deep level inthe AlN bandgap and needs to be carefully controlled if conductingcrystals are to be produced. Thus, success in creating large, conductivecrystals has proven elusive even though AlN thin films with n-typeconductivity have been demonstrated.

Furthermore, whether doped or undoped, AlN with high transparency toparticular wavelengths of light, e.g., UV light, is generally difficultto produce due to oxygen impurities and/or point defects introducedduring the fabrication process.

SUMMARY

In accordance with the present invention, a donor or acceptor level iscreated within the perfect, stoichiometric AlN or Al_(x)Ga_(1-x)N (where0≦×<1, herein sometimes referred to as AlGaN) lattice by introducing asubstitutional dopant that has greater or fewer electrons than aluminum(Al) or nitrogen (N). Charge-compensating defects, such as vacancies onthe Al cation site (designated as V_(A1)) or the N anion site(designated as V_(N)) or impurities with deep levels which will trap thefree charge created by the dopant, are desirably avoided but, moregenerally, are either reduced in density or less active. In order to useatoms that have nearly the same diameter as Al or N and avoid localstrain, dopants are preferably selected from the upper part of theperiodic table. Choices for the Al site include beryllium (Be),magnesium (Mg), zinc (Zn), carbon (C), and silicon (Si) while C, and Siare possible choices for the N site (oxygen (O) is desirably avoided forreasons detailed below). Dopants with two fewer electrons than Al, suchas lithium (Li), may also be used to make p-type AlN and AlGaN if theycan be introduced on the Al site.

Furthermore, embodiments of the invention feature methods of producinghighly transparent (e.g., to UV light) crystals of, e.g., AlN, viacontrol of oxygen content in the Al starting material and during crystalgrowth, as well as control of point-defect introduction during coolingfrom the growth temperature.

In one aspect, embodiments of the invention feature a method of formingsingle-crystal AlN including or consisting essentially of providing asubstantially undoped polycrystalline AlN ceramic having an oxygenconcentration less than approximately 100 ppm, utilizing thepolycrystalline ceramic, forming a single-crystal bulk AlN crystal by asublimation-recondensation process at a temperature greater thanapproximately 2000° C., and, after the sublimation-recondensationprocess, cooling the bulk AlN crystal to a first temperature betweenapproximately 1500° C. and approximately 1800° C. at a first rate lessthan approximately 250° C./hour (in order to, e.g., minimize formationof point defects therein).

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The bulk AlN crystal may be cooledfrom the first temperature to a second temperature lower than the firsttemperature (e.g., approximately room temperature) at a second ratefaster than the first rate. The first rate may range betweenapproximately 70° C./hour and approximately 150° C./hour.

Providing the polycrystalline AlN ceramic may include or consistessentially of cleaning a substantially undoped Al pellet and reactingthe Al pellet with nitrogen gas to form the polycrystalline AlN ceramic.The polycrystalline AlN ceramic may have the oxygen concentration ofless than approximately 100 ppm after reaching approximately roomtemperature after the reaction (e.g., without further processing by, forexample, sublimation-recondensation processes). The polycrystalline AlNceramic may not undergo any sublimation-recondensation treatment priorto the sublimation-recondensation process to form the bulk AlN crystal.A plurality of additional substantially undoped Al pellets may becleaned and reacted. Cleaning the Al pellet may include or consistessentially of exposing the undoped Al pellet to hydrofluoric acid.Cleaning the Al pellet may include or consist essentially of exposingthe undoped Al pellet to an organic solvent, exposing the undoped Alpellet to hydrochloric acid, and, thereafter, exposing the undoped Alpellet to an acid mixture including or consisting essentially of nitricacid and hydrofluoric acid (and possibly water). The absorptioncoefficient of the bulk AlN crystal may be less than approximately 20cm⁻¹ in the entire wavelength range between about 4500 nm andapproximately 215 nm. The absorption coefficient of the bulk AlN crystalmay be less than approximately 10 cm⁻¹ for the entire wavelength rangebetween approximately 400 nm and approximately 250 nm. The oxygenconcentration of the bulk AlN crystal may be less than approximately 5ppm. An AlN seed may be provided during the sublimation-recondensationprocess, and the bulk AlN crystal may form on the AlN seed. In anotheraspect, embodiments of the invention feature an AlN single crystalhaving a thickness greater than approximately 100 μm, a cross-sectionalarea greater than approximately 1 cm², and an absorption coefficient ofless than approximately 20 cm⁻¹ in the entire wavelength range betweenabout 4500 nm and approximately 215 nm. The absorption coefficient maybe less than approximately 10 cm⁻¹ for the entire wavelength rangebetween approximately 400 nm and approximately 250 nm.

These and other objects, along with advantages and features of theinvention, will become more apparent through reference to the followingdescription, the accompanying drawings, and the claims. Furthermore, itis to be understood that the features of the various embodimentsdescribed herein are not mutually exclusive and can exist in variouscombinations and permutations. As used herein, the term “substantially”means±10%, and in some embodiments, ±5%. The term “consists essentiallyof” means excluding other materials that contribute to function, unlessotherwise defined herein. The terms “undoped” and “substantiallyundoped” mean at least substantially free of intentional dopants, aseven undoped materials may incorporate slight amounts of unintentionaldopants or other impurities. Relative to steps herein describedutilizing liquid reagents (e.g., acids or organic solvents), to “expose”means to place in significant physical contact with, including but notlimited to immersing, spray-applying, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 schematically depicts a crystal growth enclosure for the growthof single-crystalline AlN;

FIGS. 2A and 2B are schematic diagrams of a furnace utilized for theformation of polycrystalline source material as described in oneembodiment of the invention;

FIGS. 3A-3E are schematic diagrams of a reactor utilized for theformation of polycrystalline source material as described in anotherembodiment of the invention;

FIG. 4 is a flowchart summarizing embodiments of techniques for forminghigh purity/high transparency AlN crystals and substrates;

FIG. 5 schematically depicts an AlN wafer separated from a boule ofsingle-crystalline AlN; and

FIG. 6 is an absorption spectrum of an AlN substrate in accordance withembodiments of the invention having high transparency in theultraviolet, visible, and infrared regions ranging from 200 nm up to2000 nm.

DETAILED DESCRIPTION

A representative environment for practice of embodiments of the presentinvention is shown in FIG. 1. AN crystals may be formed by thesublimation-recondensation method described in U.S. Pat. No. 7,638,346,the entire disclosure of which is herein incorporated by reference. Acrystal growth enclosure 100 includes a vapor mixture 110, an AlNcrystal 120, and a polycrystalline source 130, and is surrounded by afurnace 140. In an embodiment, crystal growth enclosure 100 includes orconsists essentially of tungsten. In alternative embodiments, crystalgrowth enclosure 100 includes a tungsten-rhenium alloy, rhenium, carbon,tantalum carbide, tantalum nitride, tantalum carbo-nitride, hafniumnitride, mixtures of tungsten and tantalum, or a combination thereof, asdescribed in U.S. Pat. No. 7,211,146, the entire disclosure of which ishereby incorporated by reference. Crystal growth enclosure 100 may evenconsist essentially of one or more of these materials.

Vapor mixture 110 arises from the heating of polycrystalline source 130at one end of crystal growth enclosure 100, and coalesces into AlNcrystal 120 at another, cooler end. In an embodiment, during formationof AlN crystal 120, high-purity N₂ or forming gas (i.e., a mixture ofnitrogen with hydrogen, with a hydrogen concentration ranging betweenapproximately 3% and approximately 5%) may be flowed through theenclosure 100, which may be subsequently heated to a temperature rangingbetween approximately 2000° C. and approximately 2380° C. In particular,in one embodiment, the temperature of enclosure 100 may be substantiallymaintained between about 2200° C. and about 2310° C. Polycrystallinesource 130 may be a ceramic material, may include or consist essentiallyof AlN (e.g., high-purity AlN), and may further include at least oneinterstitial or substitutional dopant. AlN crystal 120 is a bulk crystal(i.e., not a powder), may be single crystal, and may include finiteconcentrations of interstitial or substitutional dopants. In variousembodiments, AlN crystal 120 is substantially free of dopants, isextremely high purity, and exhibits high transparency, as detailedbelow. Upon further treatment, the dopants may be electrically activatedto dope AlN crystal 120 and provide it with desirable electricalproperties. In all embodiments described herein, AlN crystal 120 mayalso include gallium (Ga), rendering it an Al_(x)Ga_(1-x)N crystal. Forexample, Ga may be added to polycrystalline source 130 such that thecrystal coalesces as Al_(x)Ga_(1-x)N. In such a case, the crystal mayhave an Al concentration greater than approximately 50%. AN crystal 120may have a thickness of greater than approximately 0.1 mm and a diametergreater than approximately 1 cm. The diameter may even be greater thanapproximately 2 cm. AlN crystal 120 may be single crystalline.

The ensuing discussion describes selection of dopant species for AlNcrystal 120 (and therefore for polycrystalline source 130), as well astechniques for producing various types of polycrystalline source 130with desired properties (e.g., dopant and purity concentrations), beforereturning to the details of fabricating AlN crystal 120 and subsequentprocessing thereof.

Dopant Selection

In accordance with embodiments of the present invention, the first stepin making doped AlN crystal 120 is identifying which impurities orimpurity pairs may produce donor or acceptor centers with a smallactivation energy. For the Al site, appropriate single-element donorsmay be group IV elements such as Si, Ge, and/or transition elements suchas Ti, V, etc., while for the N site, group VI elements such as S may beused as donors. If interstitial sites are considered, lighter elements,such as H or Li, may be suitable donors. If one considers co-doping withmolecular impurities where both donors and acceptors are incorporated onthe same sublattice, then impurities such as BeSiN₂, ZnSiN₂, MgSiN₂,LiSi₂N₃, and Al₂° C. may be considered, as described in U.S. Pat. No.7,641,735, the entire disclosure of which is hereby incorporated byreference. All of these approaches desirably require the controlledintroduction of dopants into the crystal during the bulk crystal growthprocess. Two other possible dopant-generation schemes arenuclear-transmutation doping and in-diffusion of the dopant from thecrystal surface. However, these last two approaches may be moredifficult with bulk crystals thicker than approximately 2 mm becausediffusion times may be too long to be practical and the implantationenergies required may be too high. Hence, embodiments of the presentinvention preferably utilize dopants that may be introduced during bulkcrystal growth.

The next step is to select an appropriate dopant, i.e., one that canwithstand the sublimation-recondensation growth process at temperaturesup to 2330° C. (at which temperature tungsten crucibles may undergo aeutectic reaction) or hotter if an alternative crucible is used. (U.S.Pat. No. 6,719,843, the entire disclosure of which is herebyincorporated by reference, describes other possible crucible materialsfor growth of AlN bulk single crystals.) In AlN and Al_(x)Ga_(1-x)N thinepitaxial films, it has been found that Si is a shallow donor. InSi-doped GaN, n-type carrier concentrations at room temperature up to1×10²⁰/cm³ have been achieved. It has been observed to become degenerateabove about 1×10¹⁸/cm³ of Si, i.e., the conductivity is observed tobecome temperature-independent due to the high density of dopant. InSi-doped AlN, the highest room-temperature carrier concentrationobtained appears to be about 2×10²⁰/cm³. The following discussionaddresses the factors limiting Si solubility in AlN and its electricalactivation, as well as the implications for crystal growth.

The covalent radii of Al and Si are quite similar. In AlN, the averageAl—N bond distance is 1.89 Å. Thus, Si atoms are about 10% smaller thanAl atoms in these nitrides. In the pseudobinary system AlN—Si₃N₄ oneternary compound, Si₃Al₅N₉, is known. It may only exist in the presenceof oxygen as an impurity. While the solid solubility limit of Si₃N₄ inAlN (or of AlN in Si₃N₄) at room temperature or at higher temperaturesis somewhat uncertain (and is discussed below), there is ample evidenceto show that concentrations attractive to doping AlN are possible andare stable at the temperatures required for bulk crystal growth of AlN.

It has been shown that Si-doped AlN exhibits excellent blueluminescence, both photoluminescence and cathodoluminesence. This resulthas encouraged several studies of the upper limit of r, defined to bethe Si/Al atom ratio in Si-doped AlN. In analogy with typical solubilitybehavior, we expect that as the temperature increases, the solubility ofSi₃N₄ in AlN will increase.

Formation of AlN Ceramic with Controlled Dopant Concentrations

Providing a polycrystalline source 130 that includes or consistsessentially of AlN with carefully controlled dopant concentrations(including the elimination of potential deep-level impurities such asoxygen) enables growth of AlN with controllable electrical and opticalproperties. In general, oxygen is a common contaminant, and thehighest-purity AlN material that may be purchased commercially hasoxygen impurities at a level exceeding 0.3% by weight (i.e., exceeding3000 ppm by weight); therefore, references herein to dopants, dopantspecies, or intentionally introduced impurities generally exclude oxygenunless otherwise indicated. Because the vapor pressures of oxides ofaluminum are much higher than those of Al or of N₂ above contaminatedAlN, the commercial powder may be purified by heating to 2000° C. or soin a clean N₂ atmosphere. Unfortunately, the contaminated powder willtend to sinter during the heating cycle and become dense while trappingthe residual oxygen within the sintered mass. An alternative approach isto sublime (i.e., congruently evaporate Al and N₂ molecules) thecontaminated AlN in a N₂ atmosphere under a temperature gradient so thatAlN will recondense at a colder place in the furnace. Aluminumoxynitrides will condense at even colder temperatures and so higherpurity AlN ceramic will be physically separated from the aluminumoxynitride. While this process is useful in obtaining higher-purity AlN,it is time consuming and requires the dedication of a high temperaturefurnace since reasonable evaporation rates will require the furnace tobe operated at temperatures above 2200° C. In addition, it is difficultto obtain AlN with oxygen impurity concentrations below 400 ppm withthis method, probably due to the solubility of oxygen in the AlNcrystal.

Herein, oxygen concentrations are preferably measured by the TCH600Oxygen Determinator, available from LECO Corporation of St. Joseph,Mich., the Neutron Activation Analysis technique, or by dynamicsecondary ion mass spectroscopy (SIMS). The commercially available LECOmeasurement is reliable to at least a detection limit of 200 ppm byweight with normal non-inert atmosphere sample handling (surfacecontamination) of the oxygen-sensitive AlN. In addition, we have foundneutron activation to be capable of a detection limit down to at least100 ppm for polycrystalline ceramic material. For single-crystalsamples, accurate oxygen measurements may be made with dynamic SIMS,which may be calibrated using isotope implant techniques to confirm thevalidity of the measurement. All of these measurement techniques arepreferred over glow-discharge mass spectroscopy (GDMS), which isnotoriously difficult and may give erroneously low measurements.

A more efficient way to make AlN with controllable impurities is toreact, in high-purity nitrogen, either high-purity Al metal or Al metalthat is intentionally doped with the desired impurity and nothing else.In Slack and McNelly, J. Crystal Growth 34, 263 (1976), the entiredisclosure of which is hereby incorporated by reference, the problem oftrying to burn Al directly in nitrogen is described. In particular, atatmospheric pressure, the Al will react to form a protective skin aroundthe unreacted Al metal and the reaction will stop. It has beendemonstrated (M. Bockowski, A. Witek, S. Krukowski, M. Wroblewski, S.Porowski, R. M. Ayral-Marin, and J. C. Tedenac, Journal of MaterialsSynthesis and Processing, 5, 449 (1997), the entire disclosure of whichis hereby incorporated by reference) that very high nitrogen pressuresmay be used to keep the reaction going. However, the reacted AlN willform a powder and will quickly become contaminated when exposed to air.It is much more desirable to form a dense AlN material with limitedsurface area which will make it much easier to handle the AlN productwithout contaminating it.

In previous pellet-drop work by Slack and McNelly (J. Crystal Growth 42,560 (1977), the entire disclosure of which is hereby incorporated byreference), Al pellets were dropped into a pyrolytic boron nitride (pBN)crucible that is heated to about 1850° C. in an RF-heated furnace. Theindividual pellets of Al were rapidly reacted to form AlN. Slack andMcNelly obtained AlN with about 1% excess Al by weight and a residualoxygen contamination of about 400 ppm. After the AlN was formed, the pBNcrucible had to be mechanically removed and the resulting AlN had to besublimed in a temperature gradient in a nitrogen atmosphere. Slack andMcNelly used the last step (sublimation and recondensation of the AlNpolycrystalline material) to reduce the excess Al to less than 0.1%.This sublimation and recondensation step was reported to take between 12and 24 hours for 70 grams of AlN product. In accordance herewith, apellet-drop method produces AlN polycrystalline material with controlleddopant concentrations (including no dopants so that the AlN ishigh-purity) that may be used directly for AlN crystal growth withoutthe extra step of subliming and recondensing the resulting AlNpolycrystalline material. In addition, the resulting AlN polycrystallinematerial has lower oxygen contamination. Embodiments of the inventionpreferably utilize a crucible material that 1) does not react with theAlN polycrystalline material, and 2) may remain on the AlNpolycrystalline material during subsequent sublimation-recondensationgrowth of AlN single crystals from the polycrystalline material.

Highly silicon-doped AlN may be produced by burning an Al—Si alloy in anitrogen atmosphere at about 1850° C. and at 1 bar or higher pressure.At 1875° C., the N₂ dissociation pressure of Si₃N₄ is approximately 1bar, which may set a preferred upper limit on the burning temperature at1 bar since Si₃N₄ formation may result at high Si concentrations. Inthis way, some or all of the Si becomes entrapped in the AlN lattice.

The Al—Si phase diagram shows that the maximum equilibrium solidsolubility of Si in metallic Al is 1.59 atom % at 577° C. If the liquidalloys are rapidly quenched, then considerably more Si may be trapped insolid solution in the Al metal. In fact, high-purity Si-doped Al may bepurchased with Si concentrations of up to 3.5%, although it is likelythat higher concentrations of Si in Al may be obtained through rapidquenching of a molten Al and Si mixture.

Below, two examples are given of forming polycrystalline AlN materialwith controlled dopant concentrations.

Production of Doped, High-Density AlN Polycrystalline Material

Referring to FIGS. 2A and 2B, a furnace 200 may be utilized in theformation of polycrystalline source 130 incorporating a highconcentration of at least one substitutional dopant. Furnace 200includes a pellet loading mechanism 210, which drops pellets includingor consisting essentially of Al into a crucible 220. In an embodiment,the pellets may be intentionally doped with one or more dopant speciesin excess of amounts of trace impurities that may be present in pelletsconsisting essentially of Al. In an embodiment, the dopant concentrationin the pellets is greater than approximately 1% and less thanapproximately 12% by weight. In various embodiments, crucible 220includes a bottom plug 230 and a foil wrap 240. Bottom plug 230 may beapproximately cylindrical with, e.g., a diameter of approximately 0.625inches and a height of approximately 0.5 inches. Bottom plug 230 mayinclude or consist essentially of tungsten (W), or anotherhigh-melting-point material inert to AlN. Foil wrap 240 wraps aroundbottom plug 230, forming a cylinder open at the top and sealed at thebottom by bottom plug 230. Foil wrap 240 may include or consistessentially of W, or another high-melting-point material inert to AlN,and may have a thickness of approximately 0.001 inch. In an embodiment,foil wrap 240 is wrapped around bottom plug 230 multiple times, e.g., athree-ply foil wrap 240 is formed by wrapping W foil around bottom plug230 three times. Foil wrap 240 may be held in place by at least one wire250, which may include or consist essentially of a tungsten-rheniumalloy (e.g., 25% rhenium) and have a thickness of approximately 0.01inch.

Crucible 220 may be disposed within a susceptor 260 and on top of acrucible stand 270. Both susceptor 260 and crucible stand 270 mayinclude or consist essentially of W. A crucible funnel 280 may bedisposed above the top opening of crucible 220, and may include orconsist essentially of molybdenum (Mo). Crucible funnel 280 is shaped todirect pellets from pellet loading mechanism 210 into crucible 220.

Furnace 200 is typically resistively heated by one or more heatingelements 290, which are surrounded by insulation 292. Heating elements290 may be heated to temperatures up to approximately 2300° C., andfurnace 200 may operate at pressures up to approximately 60 bar. In anexemplary embodiment, the furnace operates at pressures up toapproximately 10 bar. Generally, elevated pressures may enable theincorporation of high concentrations of dopants into polycrystallinesource 130 (as described below) by limiting evaporation of the dopantspecies or a compound thereof. For example, when Si is utilized as adopant, high furnace pressures may substantially prevent the evaporationof the Si in the form of Si₃N₄. Gas flows into furnace 200 from a bottominlet 294 and is exhausted through a top outlet 296. The gas may includeor consist essentially of nitrogen or a mixture of nitrogen and 3%hydrogen (i.e., forming gas), and is generally filtered by a gas filter(not shown) that reduces levels of contaminants such as oxygen, watervapor, and hydrocarbons to less than 10 parts per billion (ppb). Anupper funnel 298 connects pellet loading mechanism 210 to cruciblefunnel 280.

In order to form doped polycrystalline source 130, pellets are desirablycleaned in preparation for loading into pellet loading mechanism 210.The pellets are preferably all similarly sized and shaped to facilitateautomatic handling (as described below). First, the pellets are siftedin order to remove oddly shaped pellets or small shavings. The pelletsare then ultrasonically cleaned in distilled water for approximately 20minutes. Next, the pellets are immersed in a mixture of hydrofluoricacid (HF) and nitric acid (HNO₃) for approximately 2 minutes at roomtemperature. Finally, the pellets are rinsed in distilled water andmultiple times in methanol, whereupon they may be stored in an inertatmosphere prior to loading into pellet loading mechanism 210. Cleaningdoped or undoped Al pellets is important to produce consistent resultsand to provide a consistent surface oxidation (or reduced layer thereof)to the reaction for production of both doped and undoped polycrystallinesource 130. In various embodiments (for both doped and undoped Alpellets), the Al pellets may be “cleaned” (i.e., have their surfaceoxidation layers removed or, at a minimum, substantially reduced, and/orhave their surfaces passivated by an oxidation-resistant layer) by theacid-based treatments described herein or by other methods, e.g., plasmatreatments.

Crucible 220 is loaded into furnace 200, and the pellets are loaded intopellet loading mechanism 210. A cleaning cycle, in which the pellets arenot dropped into crucible 220, may be run prior to an actual reactioncycle in which polycrystalline source 130 is formed. Furnace 200 isalternately subjected to a flow of forming gas and evacuated severaltimes (e.g., three times). Heating elements 290 are heated toapproximately 2200° C., thus heating crucible 220 to approximately 1950°C. Forming gas is flowed through furnace 200 at a high rate, e.g.,approximately 0.25 liters per minute (lpm) in order to purge residualmoisture and to reduce any W-containing components therein (which mayhave oxidized due to exposure to air or other sources of contamination).Heating elements 290 are then cooled back down to room temperature.

A reaction cycle is then performed to form polycrystalline source 130.Furnace 200 is alternately subjected to a flow of nitrogen and evacuatedseveral times (e.g., three times). The reaction cycle may be performedat temperatures within the range of approximately 1600° C. toapproximately 2200° C., and at pressures within the range ofapproximately 1 bar to approximately 60 bars. In an embodiment, thereaction cycle is performed at a pressure less than approximately 10bar. In an exemplary embodiment, under a nitrogen pressure ofapproximately 1.5 bars and a nitrogen flow of approximately 0.25 lpm,heating elements 290 are heated to approximately 1800° C. (correspondingto a temperature of crucible 220 of approximately 1650° C.) and held atthat temperature for approximately three hours. The gas flow isdecreased to approximately 5 standard cubic centimeters per minute(sccm), and the pellets are dropped from pellet loading mechanism 210,through upper funnel 298 and crucible funnel 280, into crucible 220. Thepellets may each weigh approximately 72 milligrams, and may be droppedat a rate of approximately 1 per minute. The pellets land on bottom plug230 (or the portion of polycrystalline source 130 already producedthereon), melt, and react with the nitrogen gas to form polycrystallinesource 130. Dopants present in the pellets are incorporated intopolycrystalline source 130 at concentrations at least partiallydetermined by the dopant concentration in the pellets and by thereaction kinetics. Very high intentional dopant concentrations inpolycrystalline source 130, e.g., greater than approximately 1% and upto approximately 12% by weight, may be achieved by using very highconcentrations of dopant in the pellet and by suppressing dopantevaporation by increasing the nitrogen pressure in reaction furnace 200.Each subsequent pellet dropped from pellet loading mechanism 210 reactsand increases the size and volume of polycrystalline source 130. In anembodiment, substantially all of each pellet reacts to formpolycrystalline source 130.

After the reaction cycle, furnace 200 (and polycrystalline source 130)is cooled down to approximately room temperature over approximately 1hour at a positive nitrogen pressure. Thus formed, polycrystallinesource 130 may weigh up to approximately 80 grams, and may include lowconcentrations of impurities such as oxygen, boron, and transitionmetals such as iron. In an embodiment, an oxygen concentration (and/orconcentration of other impurities) of polycrystalline source 130 is lessthan approximately 400 ppm by weight, and may even be less thanapproximately 100 ppm. In various embodiments, polycrystalline source130 includes or consists essentially of doped or undoped AlN that isapproximately stoichiometric, i.e., AlN that contains less thanapproximately 1% excess Al, less than approximately 0.5% excess Al, oreven less than approximately 0.1% excess Al. Polycrystalline source 130that is intentionally doped may include a concentration of a dopantspecies greater than that which may be present as a trace impurity in Aland/or AlN, e.g., greater than approximately 1% (by weight) and up toapproximately 12% (by weight) of a dopant species such as a group IVelement (e.g., Si or C), a group II element (e.g., Be or Mg) or a groupVI element (e.g., O). After formation, polycrystalline source 130 may beimmediately ready for subsequent sublimation-recondensation growth ofsingle crystal AlN, and may be stored in an inert atmosphere inpreparation therefor.

Production of High-Purity, High-Density Aln Polycrystalline Material

Referring to FIGS. 3A-3E, a reactor 300 may be utilized in the formationof polycrystalline source 130 consisting essentially of high-purity,undoped AlN. Reactor 300 includes a reaction vessel 310, which ispreferably fabricated of double-walled stainless steel and is watercooled. Reaction vessel 310 is preferably capable of a maximum internalgas pressure of approximately 45 pounds per square inch (psi), and maybe evacuated, e.g., by a turbo pump 311 (backed by a mechanical pump312) to approximately 10⁻⁷ Torr. A feeder mechanism 320 is connected tothe top of reaction vessel 310, and may be evacuated and pressurizedwith the same gases and pressures as reaction vessel 310. Feedermechanism 320 may be isolated from reaction vessel 310 by an isolationvalve 322. Pellets (which may consist essentially of high- (e.g., fivenines) purity undoped Al and may be shaped approximately cylindrically)released from feeder mechanism 320 are directed to a crucible 330 by anupper funnel 332 and a lower funnel 334.

Crucible 330 includes a bottom plug 336 and a foil wrap 337. Bottom plug336 may be approximately cylindrical with, e.g., a diameter ofapproximately 2 inches and a height of approximately 0.5 inches. Bottomplug 336 may include or consist essentially of W, or anotherhigh-melting-point material inert to AlN. Foil wrap 337 wraps aroundbottom plug 336, forming a cylinder open at the top and sealed at thebottom by bottom plug 336. Foil wrap 337 may include or consistessentially of W, or another high melting point material inert to AlN,and may have a thickness of approximately 0.001 inch. In an embodiment,foil wrap 337 may be wrapped around bottom plug 336 multiple times,e.g., a three-ply foil wrap 337 is formed by wrapping W foil aroundbottom plug 337 three times. Foil wrap 337 may be held in place by wire338. Wire 338 may include or consist essentially of a tungsten-rheniumalloy (e.g., 25% rhenium) and have a thickness of approximately 0.01inch.

Crucible 330 is typically disposed within a reaction zone 340 and on topof a crucible stand 342. Both reaction zone 340 and crucible stand 342may include or consist essentially of W. Lower funnel 334 is disposedabove the top opening of crucible 330, and may include or consistessentially of W. Lower funnel 334 is typically shaped to direct pelletsfrom feeder mechanism 320 and upper funnel 332 into crucible 330.

Reactor 300 includes an inductive heating coil 350, which wraps aroundinsulation 360. Insulation 360 may include or consist essentially ofbubble alumina available from Zircar Ceramics, Inc. of Florida, N.Y.,held within a quartz holder. Inductive-heating coil 350 may be a 10 kHz,20 kilowatt inductive-heating system available from Mesta Electronics,Inc. of N. Huntingdon, Pa., and may heat to temperatures up toapproximately 2300° C. An optical pyrometer port 362 enables themeasurement of temperature inside the reaction zone defined byinductive-heating coil 350 by pyrometry. Gas from a series of gas tanksrepresentatively indicated at 368 flows into reactor 300 from a bottominlet 370 and/or a top inlet 372. The gas may include or consistessentially of nitrogen or forming gas, and is generally filtered by agas filter 374 that reduces levels of contaminants such as oxygen, watervapor, and hydrocarbons to less than 10 ppb. A vertical drive 380 may beused to move crucible 330 in and out of the hot zone created byinductive heating coil 350. A conventional control station 390 includeselectronic controls and power supplies for all of the componentsassociated with reactor 300.

With reference to FIG. 4, in order to form undoped polycrystallinesource 130, pellets are preferably cleaned in preparation for loadinginto feeder mechanism 320, as indicated in steps 400 and 410. In variousembodiments, the pellets are sifted (with or without water) in order toremove oddly shaped pellets or small shavings. The pellets are thenultrasonically cleaned in an organic solvent (e.g., methanol) forapproximately 20 minutes, etched for approximately 7 minutes inhydrochloric acid (HCl), and rinsed several times (e.g. three times) indistilled water. After another ultrasonic clean in, e.g., methanol, forapproximately 20 minutes, the pellets are immersed in a mixture of HFand HNO₃ for approximately 2 minutes at room temperature. Finally, thepellets are rinsed in distilled water and multiple times in, e.g.,methanol, whereupon they may be stored in an inert or nitrogenatmosphere prior to loading in feeder mechanism 320.

Crucible 330 is loaded into reactor 300, and pellets are loaded intofeeder mechanism 320. Reaction chamber 310 and feeder mechanism 320 areevacuated, e.g., to a pressure less than approximately 5×10⁻⁵ Torr, andrefilled with forming gas to a pressure of approximately 6 psi. Eithernitrogen (N₂) gas or forming gas flows into reaction chamber 310 frombottom inlet 370 and top inlet 372 at a rate of, e.g., approximately0.25 lpm. The flow of gas provides a sufficient amount of nitrogen inreaction chamber 310 to convert the pellet(s) to AlN (as describedbelow). Inductive heating coil 350 heats crucible 330 to approximately1900-2200° C., but even higher temperatures may be utilized. In apreferred embodiment, inductive heating coil 350 heats crucible 330 toapproximately 2000-2050° C. Temperatures in this range have been foundto be sufficient to totally react the pellets into stoichiometric AlN(which includes less than approximately 1% unreacted Al, less thanapproximately 0.5% unreacted Al, or even less than approximately 0.1%unreacted Al) and to drive off higher-vapor-pressure impurities that maybe trapped within polycrystalline source 130 and create opticalabsorptions. The temperature at crucible 330 may be measured bypyrometry through optical pyrometer port 362. Once crucible 330 reachesthe desired temperature, the temperature and gas flow conditions withinreactor 300 are held constant for an approximately three-hour pre-soakcycle. The pre-soak cleans crucible 330 and other parts of reactor 300of contaminants, e.g., oxides, before the introduction of the Alpellets.

A reaction cycle is then performed to form undoped polycrystallinesource 130. Pellets are dropped from feeder mechanism 320, through upperfunnel 332 and lower funnel 334, into crucible 330. The pellets may eachweigh approximately 0.23 gram, and may be dropped at a rate ofapproximately one every 90 seconds. Feeder mechanism 320 may incorporatean optical counter that counts actual pellet drops and may cycle feedermechanism 320 to drop an additional pellet in case of a loading error.The pellets land on bottom plug 336 (or the portion of polycrystallinesource 130 already produced thereon), melt, and react with the nitrogengas to form undoped polycrystalline source 130, as indicated in step 420of FIG. 4. Each subsequent pellet dropped from feeder mechanism 320reacts and increases the size and volume of polycrystalline source 130.In an embodiment, substantially all of each pellet reacts to formpolycrystalline source 130. After a desired number of pellets arereacted to form polycrystalline source 130, the reaction-gas flow rateand temperature are maintained for approximately 1 hour to ensure thatthe reaction is complete.

After the reaction cycle, crucible 330 (and polycrystalline source 130)is cooled down to approximately room temperature over, e.g.,approximately 1 hour at a positive nitrogen pressure. Thus formed,polycrystalline source 130 may weigh up to approximately 155 grams, andconsists essentially of high-purity, undoped AlN. In an embodiment, anoxygen concentration (and/or concentration of other impurities such asboron or transition metals) of polycrystalline source 130 is less thanapproximately 400 ppm by weight, and may even be less than approximately100 ppm. Polycrystalline source 130 includes or consists essentially ofundoped AlN that is approximately stoichiometric, i.e., AlN thatcontains less than approximately 1% excess Al, less than approximately0.5% excess Al, or even less than approximately 0.1% excess Al. Afterformation, polycrystalline source 130 may be immediately ready forsubsequent sublimation-recondensation growth of single crystal AlN, andmay be stored in an inert atmosphere in preparation therefor.

Formation of Single-Crystal AlN

Once doped or undoped polycrystalline source 130 has been fabricated byone of the techniques described above with reference to FIGS. 2A and 2Band 3A-3E, it may be utilized in the sublimation-recondensation growthof single-crystal AlN as described above with reference to FIG. 1.Because polycrystalline source 130 is generally approximatelystoichiometric AlN with low concentrations of impurities, it may be usedto form AlN crystal 120 without further preparation (e.g., withoutintermediate sublimation-recondensation steps). Polycrystalline source130 is separated from bottom plug 230 (or bottom plug 336), but foilwrap 240 (or foil wrap 337) typically remains proximate and in contactwith polycrystalline source 130. Foil wrap 240 (or foil wrap 337) mayremain in contact with polycrystalline source 130 and placed in crystalgrowth enclosure 100. Since foil wrap 240 (or foil wrap 337) is formedof W or other material inert to AlN, it does not react with orcontaminate AlN crystal 120 during its formation. In an embodiment,polycrystalline source 130, surrounded by foil wrap 240 (or foil wrap337) may be broken into smaller pieces, and one or more of them may beutilized separately to form AlN crystal 120. In this embodiment, piecesof foil wrap 240 (or foil wrap 337) may remain in contact with thepieces of polycrystalline source 130. In another embodiment, foil wrap240 (or foil wrap 337) may be formed of the same material as crystalgrowth enclosure 100, e.g., W. As indicated in steps 430 and 450 of FIG.4, at least a portion of polycrystalline source 130 (e.g., with orwithout foil wrap 240 or foil wrap 337) is placed in crystal growthenclosure 100 for formation of AlN crystal 120 by, e.g.,sublimation-recondensation (as described above). One or more seeds(e.g., that include or consist essentially of AlN) may also be placedwithin crystal growth enclosure 100 in various embodiments (as indicatedin step 440 of FIG. 4), and in such embodiments, AlN crystal 120nucleates and grows on the seed(s). Alternatively, AlN crystal 120 maybe formed without a seed, as described above.

The AlN crystal 120 at this point may have an absorption coefficientless than approximately 20 cm⁻¹ in the entire wavelength range betweenabout 4500 nm and approximately 215 nm, and preferably less than about10 cm⁻¹ for the entire wavelength range between approximately 400 nm andapproximately 250 nm. The oxygen concentration (and/or concentrations ofother impurities) may be less than approximately 50 ppm by weight, oreven less than approximately 5 ppm by weight. As detailed in FIG. 4,additional cooling techniques may be applied to the AlN crystal 120 tofacilitate its retention of a coefficient of absorption no more thanabout 20 cm⁻¹ in the entire range between about 4500 nm andapproximately 215 nm (as also shown in FIG. 6).

As indicated in step 460 of FIG. 4, the AlN crystal 120 may be cooledfrom the growth temperature at a controlled rate for an initial periodof time, e.g., until AlN crystal 120 reaches a temperature ofapproximately 1800° C. In this manner, the formation of light-absorbingpoint defects within AlN crystal 120 may be markedly reduced, enablingretention of the above-described low absorption coefficients. In variousembodiments, the cooling rate may be less than approximately 250° C./hr.The cooling rate may even range between approximately 150° C./hr andapproximately 70° C./hr from the growth temperature down toapproximately 1800° C., or even down to approximately 1500° C. Forexample, in an embodiment where the growth temperature is about 2300°C., this initial cooling period may be approximately two hours.

Following the initial cooling period, the temperature of AlN crystal 120is generally at about 1500° C.-1800° C. By slowing the cool down of AlNcrystal 120 from growth temperature for this initial period, theformation of light-absorbing point defects which may lead to undesirableabsorption bands is substantially avoided. Of particular note,absorption bands in the 300 nm-350 nm range may be avoided as a resultof the described controlled cooling in the initial period. FIG. 6depicts an exemplary measured absorption spectrum for high-purity AlNfabricated in accordance with embodiments of the present invention. Asshown, after the controlled cooling, AlN crystal may have an absorptioncoefficient below approximately 10 cm⁻¹ for the entire wavelength rangebetween 300 nm and 350 nm. By contrast, AlN crystals cooled at a fasterrate in the above-described temperature regime generally accumulate highdensities of point defects that result in higher absorption coefficientsin the above-described wavelength ranges. For example, after controlledcooling, the optical absorption in the above wavelength range may bereduced by as much as 15× (due to lower concentrations of point defects)compared to that obtained with an uncontrolled cooling rate.

With continued reference to FIG. 4, controlled cooling of AlN crystal120 from the growth temperature may also have the additional advantageof minimizing or eliminating deleterious cracking. As a result, AlNcrystal 120 may have a diameter of, e.g., two or more inches asdescribed above, without significant concern over compromise to thestructural integrity or transparency of the boule. Additionally, afterapproximately two hours or cooling, and/or after cooling to atemperature of less than approximately 1800° C. to approximately 1500°C., the temperature of AlN crystal 120 may be reduced at a greater ratewithout significant concern over the formation of such defects. That is,as indicated in step 470 of FIG. 4, AlN crystal 120 may be allowed tocool to, e.g., approximately room temperature at a faster rate than theabove-described initial controlled rate, e.g., a rate resulting fromcooling without a controlled application of heat to slow the cooling.Allowing AlN crystal 120 to cool at this subsequent faster rate maysubstantially reduce processing time and provide significant costsavings in terms of process efficiency.

Referring to FIG. 5, after formation of AlN crystal 120, wafer 500 maybe separated from AlN crystal 120 by the use of, e.g., a diamond annularsaw or a wire saw, as also indicated in step 480 of FIG. 4. In anembodiment, a crystalline orientation of wafer 500 may be withinapproximately 2° of the (0001) face (i.e., the c-face). Such c-facewafers may have an Al-polarity surface or an N-polarity surface, and maysubsequently be prepared as described in U.S. Pat. No. 7,037,838 or U.S.Patent Application Publication No. 2006/0288929, the entire disclosuresof which are hereby incorporated by reference. In other embodiments,wafer 500 may be oriented within approximately 2° of an m-face or a-faceorientation (thus having a non-polar orientation) or may have asemi-polar orientation if AlN crystal 120 is cut along a differentdirection. The surfaces of these wafers may also be prepared asdescribed in U.S. Pat. No. 7,037,838. Wafer 500 may have a roughlycircular cross-sectional area with a diameter of greater thanapproximately 2 cm. In an alternate embodiment, a surface area of wafer500 may be greater than approximately 1 cm², or even greater thanapproximately 3 cm², and may be shaped like a quadrilateral or otherpolygon. A thickness of wafer 500 may be greater than approximately 100μm, greater than approximately 200 μm, or even greater thanapproximately 2 mm. Wafer 500 preferably has the properties of AlNcrystal 120, as described herein. For example, the oxygen concentrationof wafer 500 sliced from AlN crystal 120 prepared with high-puritypolycrystalline source 130 may be less than approximately 5×10¹⁷ cm⁻³(i.e., less than approximately 5 ppm per weight), as measured, forexample, by SIMS.

When a doped polycrystalline source 130 including a dopant species isused to form AlN crystal 120, AlN crystal 120 and wafer 500 may bothincorporate the dopant species at a concentration greater thanapproximately 10¹⁶/cm³. Depending on the particular dopant species, AlNcrystal 120 and/or wafer 500 may exhibit n-type or p-type conductivity.In an embodiment, an oxygen concentration (and/or concentration of otherimpurities such as boron or transition metals such as iron) of AlNcrystal 120 and/or wafer 500 is less than approximately 400 parts permillion by weight (ppm), and may even be less than approximately 100ppm, or even less than approximately 50 ppm. The oxygen concentrationmay also be less than approximately 4.5×10¹⁹/cm³, or even less than1×10¹⁹/cm³, as measured by dynamic SIMS. A conductivity of AlN crystal120 and/or wafer 500 at room temperature may be greater thanapproximately 10⁻⁴ Ω⁻¹ cm⁻¹, or even greater than approximately 10⁻² Ω⁻¹cm⁻¹. A thermal conductivity of AlN crystal 120 and/or wafer 500 may begreater than approximately 270 Watts per meter-Kelvin (W/m·K), a valuepreferably measured by the American Society for Testing and Materials(ASTM) Standard E1461-01 (Current Industry Standard Test Method forThermal Diffusivity of Solids by the Laser Flash Method), and providedby a commercial vendor such as NETZSCH Inc. of Exton, Pa.

Vapor Pressures of AlN and Si₃N₄

The relative Si, Al, and N₂ vapor pressures as a function of temperaturemay strongly affect growth of Si-doped AlN crystals. These vaporpressures may be calculated from the JANAF tables (M. W. Chase, Jr.,Journal of Physical and Chemical Reference Data, Monograph No. 9,NIST-JANAF Thermochemical Tables, Fourth edition (1998)), the entiredisclosure of which is hereby incorporated by reference. The AlNevaporates congruently as Al atoms and N₂ molecules with very smalltraces of Al₂ and AlN vapor molecules. If there is any Si₃N₄ in thesource, then, at a temperature of 2300° C. that is typically used forgrowing AlN, the nitrogen pressure over Si₃N₄ solid is 53 bars.Accordingly, the decomposition pressure of solid Si₃N₄ is substantiallyhigher than that of solid AlN. When a small amount of either Si or Si₃N₄is dissolved in AlN, however, the Si vapor pressure is much reduced. Ifthe crystal composition is AlN_(1-x)Si_(x), then the total pressure ofSi in the gas phase is roughly P(Si)=x×P(Al). This is due to the factthat the Si to Al ratio in the equilibrium gas phase is the same as inthe solid.

The partial pressure of Al vapor over AlN in 1 bar of N₂ at 2300° C. is0.09 bars. If x is 0.10, then P(Si)=0.009 bar. This is about the same asthe Si partial pressure over Si₃N₄ at this temperature, which is 0.008bar. During crystal growth, the N₂ pressure over the AlN is typicallykept between 0.5 and 10 bar, with 1.2 bar being preferred. This nitrogenpressure is much less than the N₂ pressure of 53 bars needed to formsolid Si₃N₄. Thus, no Si₃N₄ is formed under these conditions. The Siatoms are transported to the growing Al crystal as mostly Si₁ atoms(over 80%) although some transport may be expected as SiN, Si₂N, Si₂ andSi₂ gas-phase molecules. Undoped AlN will grow very close tostoichiometric. The nitrogen vacancy (V_(N)) concentration depends onthe growth temperature and the nitrogen pressure; for growth at 2300°C., the aluminum nitride grows as Al₁N_(1-y)(V_(N))_(y), where y may be˜10⁻⁴ at 1 bar N₂.

Silicon-Doped AlN Crystals

After making silicon-doped AlN ceramic by reacting Al—Si alloys withnitrogen, this material may be used to grow crystals by theevaporation-recondensation or solid-gas-solid technique. Tungstencrucibles are typically employed for growing AlN; as explained herein,the same crucibles may be used for growing Si-doped AlN crystals if thenitrogen pressure is between 0.5 to 10 bar and the temperature is 2000°C. to 2300° C. The Si—W system possesses two intermediate compounds:WSi₂ (melting point (m.p.) 2160° C.) and W₅Si₃ (m.p. 2320° C.). Thepartial pressure of Si in the gas phase is preferably maintained lowenough to prevent the formulation of these phases at the growthtemperature. For Si/Al ratios of up to 0.1 (10%), substantially noreaction of Si with the tungsten should occur although there may be someabsorption of Si by a tungsten crucible.

Thus, to achieve higher doping levels and/or to increase the fraction ofSi that is captured in the growing crystal from a Si-doped AlN ceramic,it may be desirable to use a crucible constructed of an alternativematerial. See, e.g., G. A. Slack, J. Whitlock, K. Morgan, and L. J.Schowalter, Mat. Res. Soc. Proc. 798, Y10.74.1 (2004), the entiredisclosure of which is hereby incorporated by reference. In anembodiment a TaC crucible (e.g., prepared as described in U.S. Pat. No.7,211,146, the entire disclosure of which is hereby incorporated byreference) is used, as it may not react with either the AlN or Si₃N₄,nor with the Al and Si vapors, in the temperature range of approximately1800° C. to approximately 2300° C. and nitrogen pressures fromapproximately 1 bar to 60 bars.

Treated and Untreated Crystals

In analogy with the AlN—Al₂O₃ system where Al₂O₃ plus an Al vacancyenters the AlN lattice as Al₂V_(Al)O₃, at high concentrations of Si, oneexpects to obtain a mixed crystal of AlN—Si₃V_(A1)N₄, with each Si₃N₄molecule introducing one Al atom vacancy. Unfortunately, theintroduction of Al vacancies will generally introduce acceptor levelswhich will compensate the Si donor levels. Thus, it is desirable tosuppress the formation of Si₃V_(Al)N₄ in the AlN crystal.

In thin epitaxial layers of AlN grown on diamond substrates with Sidoping in this range by R. Zeisel, et al., Phys. Rev. B61, R16283(2000), the entire disclosure of which is hereby incorporated byreference, the apparent activation energy for conduction was shown tovary from about 100 to 600 meV with the material becoming lessconducting as the Si concentration increases. Zeisel et al. suggestedthat Si impurities in AlN to form a DX-center that has a high activationenergy. However, C. G. Vande Walle, Phys. Rev. B57 R2033 (1998) and C.G. Vande Walle, et al. MRS Internet J. Nitride Semicond. Res. 4S1, G10.4(1999), the entire disclosures of which are hereby incorporated byreference, have shown that Si in AlN does not form such centers andtypically stays centered on an Al lattice site. The decrease observed inthe electrical activity of the Si by Zeisel et al. may be caused by anincreasing concentration of Al vacancies as the Si content increases.This agrees with the simple idea that Si atoms enter the AlN lattice asSi₃V_(Al)N₄ in order to maintain charge neutrality. Here V_(Al)designates an aluminum atom vacancy. In accordance with this lattermodel, then one may activate the Si by generating nitrogen vacancies inthe AlN lattice. The nitrogen vacancies will tend to convert the Si₃N₄to SiN by combining with aluminum vacancies to form voids. When theconversion is complete, nearly all of the Si atoms are typicallyelectrically active.

Doped AlN crystals are typically grown under conditions which generatenitrogen vacancies as described above. However, it is also possible toanneal AlN crystal 120 after growth by reducing the nitrogen partialpressure in vapor mixture 110 above the crystal while keeping crystalgrowth enclosure 100 in nearly an isothermal environment at atemperature above 1800° C., as described above. Crystal growth enclosure100 may be made from W, but alternative crucible materials, such as TaC,may be preferred to reduce the loss of dopant (e.g., Si) through itswalls.

If AlN crystals with Si concentrations greater than 1.3×10²¹ cm⁻³ aregrown, then, according to Hermann et al., Appl. Phys. Letters 86 192108(2005), the entire disclosure of which is hereby incorporated byreference, the crystals will be electrically degenerate and theresistivity can be as low as 2 to 3 Ω-cm at room temperature. Even lowerresistivities may result if the formation of V_(A1) is suppressed asdescribed herein.

Annealing Treatments

Annealing treatments may be employed as a means of controlling thenitrogen vacancy content, aluminum vacancy content, and/or dopantelectrical activation in wafer 500 cut from AlN crystal 120. ExemplaryAlN crystal 120 doped with Si is grown at nitrogen pressures between 0.5and 10 bars; lower N₂ pressures may significantly slow the growth rateor suppress it entirely. Once grown, however, some of the nitrogen maybe extracted from AlN crystal 120 or wafer 500, i.e., nitrogen vacanciesmay be injected into the material. Wafer 500 may be annealed at atemperature greater than approximately 1900° C. in order to electricallyactivate a dopant species therein. The annealing may also decrease aconcentration of Al vacancies and/or increase a concentration of Nvacancies in wafer 500.

As the N₂ pressure is reduced around the crystal at approximately 1900°C. or above, the nitrogen diffuses out. In an embodiment, a suitable N₂pressure at temperature T for creating the maximum number of N vacanciesis greater than the pressure required to form AlN from Al at the sametemperature T. For example, for an annealing temperature of 2000° C., asuitable N₂ pressure used during annealing may be selected from therange of approximately 2 millibar (mbar) to approximately 0.5 bar. TheN₂ pressure may be less than approximately twice a N₂ pressure requiredto form AlN from Al at temperature T. In an embodiment, the N₂ pressureused during annealing is selected from the range of approximately 0.1mbar and approximately 5 bars. Higher pressures may generally bepreferred at higher temperatures. In another embodiment, an inert gassuch as argon (Ar) is introduced during the annealing to suppress Alevaporation as discussed below. The reduction of “Si₃N₄” in solution inthe AlN crystal to form SiN happens before the decomposition of Al₁N₁ toAl₁N_(1-ε), where ε is the nitrogen loss expected in undoped AlN. The Alvacancies and nitrogen vacancies may combine and be replaced by latticevoids or surface pits and the chemical composition becomes Al_(1-x)Si_(x) N₁. Essentially the Si is now present as SiN and is thuselectrically active as a donor. When it was present as Si₃N₄, it waselectrically inactive. After annealing, substantially all of a dopantspecies (such as Si) present in wafer 400 may be electrically activated.A conductivity of annealed wafer 400 may be greater than approximately10⁻⁴ Ω⁻¹ cm⁻¹, or even greater than approximately 10⁻² cm⁻¹ at roomtemperature.

Making N-Type AlN Using Only Nitrogen Vacancies

Annealing may generate enough nitrogen vacancies in undoped AlN so thatthe electron donors are the excess Al atoms. In this case, one mayanneal undoped wafer 500 in a low N₂ gas pressure at temperaturesbetween 1700° C. and 2200° C. During the anneal, some of the nitrogen inthe AlN will diffuse out to the surface and escape, leaving the Albehind. This is preferably done in a flowing argon-nitrogen atmosphereat a total pressure within the range of approximately 2 bars toapproximately 30 bars. The argon prevents the Al from evaporating. Thenitrogen pressure is just enough to keep the AlN from converting back tometallic aluminum. That is, the pressure of N₂ is greater than the N₂pressure required to form AlN from Al at the anneal temperature.Annealed undoped wafer 500 (consisting essentially of AlN with noextrinsic electron-donating dopants) may have a conductivity greaterthan approximately 10⁻² Ω⁻¹ at room temperature. Such conductivity maybe supplied by excess Al atoms (equivalently, by nitrogen vacancies) inthe AlN lattice.

It will be seen that the techniques described herein provide a basis forproduction of undoped and doped crystals including AlN and AlGaN.

The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:
 1. A method of forming single-crystal AlN, themethod comprising: providing a substantially undoped polycrystalline AlNceramic having an oxygen concentration less than approximately 100 ppmwithin a crystal growth enclosure, the crystal growth enclosure beingdisposed within a furnace; utilizing the polycrystalline AlN ceramic,forming a single-crystal bulk AlN crystal by asublimation-recondensation process at a growth temperature greater thanapproximately 2000° C.; and after the sublimation-recondensationprocess, (i) cooling the bulk AlN crystal from the growth temperature toa first temperature between approximately 1500° C. and approximately1800° C. at a first rate ranging from 70° C./hour to 150° C./hour, thefirst rate being slowed via controlled application of heat from thefurnace, and (ii) thereafter, cooling the bulk AlN crystal from thefirst temperature to a second temperature lower than the firsttemperature at a second rate greater than approximately 250° C./hour. 2.The method of claim 1, wherein the second temperature is approximatelyroom temperature.
 3. The method of claim 1, wherein providing thepolycrystalline AlN ceramic comprises: cleaning a substantially undopedAl pellet; and reacting the Al pellet with nitrogen gas to form thepolycrystalline AlN ceramic, wherein the polycrystalline AlN ceramic hasthe oxygen concentration of less than approximately 100 ppm afterreaching approximately room temperature after the reaction.
 4. Themethod of claim 3, wherein (i) the polycrystalline AlN ceramic has lessthan 1% excess Al after reaching approximately room temperature afterthe reaction, and (ii) the polycrystalline AlN ceramic does not undergoany sublimation-recondensation treatment prior to thesublimation-recondensation process to form the bulk AlN crystal.
 5. Themethod of claim 3, further comprising cleaning and reacting a pluralityof additional substantially undoped Al pellets.
 6. The method of claim3, wherein cleaning the undoped Al pellet comprises exposing the undopedAl pellet to hydrofluoric acid.
 7. The method of claim 3, whereincleaning the undoped Al pellet comprises: exposing the undoped Al pelletto an organic solvent; exposing the undoped Al pellet to hydrochloricacid; and thereafter, exposing the undoped Al pellet to an acid mixturecomprising nitric acid and hydrofluoric acid.
 8. The method of claim 1,wherein an absorption coefficient of the bulk AlN crystal is less thanapproximately 20 cm⁻¹ in the entire wavelength range between about 4500nm and approximately 215 nm.
 9. The method of claim 1, wherein anabsorption coefficient of the bulk AlN crystal is less thanapproximately 10 cm⁻¹ for the entire wavelength range betweenapproximately 400 nm and approximately 250 nm.
 10. The method of claim1, wherein an oxygen concentration of the bulk AlN crystal is less thanapproximately 5 ppm.
 11. The method of claim 1, wherein at least aportion of the polycrystalline AlN ceramic is in contact with a foilwrap during the sublimation-recondensation process, the foil wrapcomprising a material inert to AlN.
 12. The method of claim 11, whereinthe foil wrap consists essentially of W.
 13. The method of claim 11,wherein the foil wrap and the crystal growth enclosure comprise the samematerial.
 14. The method of claim 1, wherein the second rate resultsfrom cooling without controlled application of heat from the furnace.15. The method of claim 1, further comprising providing an AlN seedduring the sublimation-recondensation process, wherein the bulk AlNcrystal forms on the AlN seed.
 16. A method of forming single-crystalAlN, the method comprising: providing a polycrystalline AlN ceramicwithin a crystal growth enclosure, the crystal growth enclosure beingdisposed within a furnace; utilizing the polycrystalline AlN ceramic,forming a single-crystal bulk AlN crystal by asublimation-recondensation process at a growth temperature greater thanapproximately 2000° C.; and after the sublimation-recondensationprocess, (i) cooling the bulk AlN crystal from the growth temperature toa first temperature between approximately 1500° C. and approximately1800° C. at a first rate ranging from 70° C./hour to 150° C./hour, thefirst rate being slowed via controlled application of heat from thefurnace, and (ii) thereafter, cooling the bulk AlN crystal from thefirst temperature to a second temperature lower than the firsttemperature at a second rate greater than approximately 250° C./hour.17. The method of claim 16, wherein the polycrystalline AlN ceramic issubstantially undoped.
 18. The method of claim 16, wherein an oxygenconcentration of the polycrystalline AlN ceramic is less thanapproximately 100 ppm.
 19. The method of claim 16, wherein the secondtemperature is approximately room temperature.
 20. The method of claim16, wherein providing the polycrystalline AlN ceramic comprises:cleaning an Al pellet; and reacting the Al pellet with nitrogen gas toform the polycrystalline AlN ceramic, wherein the polycrystalline AlNceramic has an oxygen concentration of less than approximately 100 ppmafter reaching approximately room temperature after the reaction. 21.The method of claim 20, wherein (i) the polycrystalline AlN ceramic hasless than 1% excess Al after reaching approximately room temperatureafter the reaction, and (ii) the polycrystalline AlN ceramic does notundergo any sublimation-recondensation treatment prior to thesublimation-recondensation process to form the bulk AlN crystal.
 22. Themethod of claim 20, further comprising cleaning and reacting a pluralityof additional Al pellets.
 23. The method of claim 20, wherein cleaningthe Al pellet comprises exposing the Al pellet to hydrofluoric acid. 24.The method of claim 20, wherein cleaning the Al pellet comprises:exposing the Al pellet to an organic solvent; exposing the Al pellet tohydrochloric acid; and thereafter, exposing the Al pellet to an acidmixture comprising nitric acid and hydrofluoric acid.
 25. The method ofclaim 16, wherein an absorption coefficient of the bulk AlN crystal isless than approximately 20 cm⁻¹ in the entire wavelength range betweenabout 4500 nm and approximately 215 nm.
 26. The method of claim 16,wherein an absorption coefficient of the bulk AlN crystal is less thanapproximately 10 cm⁻¹ for the entire wavelength range betweenapproximately 400 nm and approximately 250 nm.
 27. The method of claim16, wherein an oxygen concentration of the bulk AlN crystal is less thanapproximately 5 ppm.
 28. The method of claim 16, wherein at least aportion of the polycrystalline AlN ceramic is in contact with a foilwrap during the sublimation-recondensation process, the foil wrapcomprising a material inert to AlN.
 29. The method of claim 28, whereinthe foil wrap consists essentially of W.
 30. The method of claim 28,wherein the foil wrap and the crystal growth enclosure comprise the samematerial.
 31. The method of claim 16, wherein the second rate resultsfrom cooling without controlled application of heat from the furnace.32. The method of claim 16, further comprising providing an AlN seedduring the sublimation-recondensation process, wherein the bulk AlNcrystal forms on the AlN seed.
 33. The method of claim 16, wherein thepolycrystalline AlN ceramic is doped with a non-oxygen dopant.
 34. Themethod of claim 33, wherein the dopant comprises at least one ofberyllium, magnesium, zinc, carbon, silicon, or lithium.
 35. The methodof claim 33, wherein the bulk AlN crystal comprises the dopant at aconcentration greater than approximately 10¹⁶/cm³.